Aristotle's definition of time as 'a number of motion with respect to the before and after' has been branded as patently circular by commentators ranging from Simplicius to W. D. Ross. In this book Tony Roark presents an interpretation of the definition that renders it not only non-circular, but also worthy of serious philosophical scrutiny. He shows how Aristotle developed an account of the nature of time that is inspired by Plato while also thoroughly bound up with Aristotle's sophisticated analyses of motion and perception. When Aristotle's view is properly understood, Roark argues, it is immune to devastating objections against the possibility of temporal passage articulated by McTaggart and other 20th-century philosophers. Roark's novel and fascinating interpretation of Aristotle's temporal theory will appeal to those interested in Aristotle, ancient philosophy and the philosophy of time.